The search for renewable energy sources which do not produce waste and are both ecologically acceptable and economically poses one of the greatest challenges of this era. One solution to the problem of providing "clean" energy is solar energy. Numerous types of solar energy collectors have been developed to transform solar energy into an energy form which is easier to store and transport, such as electrical energy, for example. Photovoltaic cells, currently called solar cells, function essentially according to the following principle: when a photon reaches a semi-conductor, it generates charge carriers by promoting electrons in the valence shell to the conduction shell and thus produces electron/hole pairs. Thus, an electromotive force is present on opposite sides of the junction and the semiconductor acts as an energy source.
Two methods of making such photovoltaic cells have been explored. One consists of using a crystalline material with high voltaic efficiency (greater than 10%) cut into plates. The other consists of depositing a thin layer of material with a lower efficiency (5% to 10%) on a large, inexpensive support (glass, stainless, plastic, etc.). This invention involves the former method.
Various methods have been developed for making photovoltaic cells from crystalline materials having high voltaic efficiency. However, these methods are not without shortcomings in terms of expensive fabrication costs and marginal outputs from the cells.
A first method of producing such cells consists of using thermal diffusion to dope a silicon substrate with an element such as boron or phosphorus at a temperature above 1,000.degree. C. This high temperature treatment consumes much energy and thus is costly; additionally, when a thin substrate is used at such a high temperature, it tends to break or bend. Consequently, solar cells produced by this method are quite expensive.
Other procedures have been explored to overcome these disadvantages. One of these consists of replacing the doping process with the deposition of a doped layer at a lower temperature. This is accomplished by depositing a thin layer of amorphous silicon of p type conductivity on a crystalline silicon substrate of n type conductivity at a temperature of less than 200.degree. C. Accordingly, a heterojunction of the type p/n is obtained. Since it takes place at a lower temperature, this procedure consumes relatively little energy. In addition, the lower temperature reduces cross-contamination of the charge carriers caused by diffusion of impurities from the treatment chamber, a fact which improves cell output. However, the output is still marginal, as cross-contamination is not entirely eliminated.
In an attempt to eliminate this phenomena altogether, a procedure has been devised whereby an intrinsic layer of amorphous silicon is deposited between the crystalline silicon layer having n type conductivity and the amorphous silicon layer having p type conductivity. This cell, known as ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin layer), is fabricated using a low temperature method and provides an efficient output, since there is no cross-contamination and the p/n type junction is abrupt.
Further attempts to increase photovoltaic cell efficiency have resulted in the development of semiconductive junctions on both substrate surfaces. To achieve this, the cell has a silicon substrate with p type conductivity, to which is added a front layer with n type conductivity and a silicon dioxide passivation layer (SiO.sub.2). This cell is fabricated as follows: first, the silicon dioxide passivation layer is deposited on the rear surface of the substrate; then, a portion of this passivation layer is removed; and finally, the positive conductive layer is deposited, doped with boron, to form the rear contact by locally creating a back surface field. Using a silicon dioxide passivation layer necessarily implies the creation of non-passive zones to form the rear contact. While this method has achieved the maximum cell efficiency to date, it also has certain drawbacks. It is relatively difficult to diffuse the boron while simultaneously maintaining the stability of the charge carriers at an output sufficient to result in high cell efficiency. Furthermore, to achieve an effective junction, the boron diffusion must be substantial and therefore must occur over a fairly long period of time and at a high temperature. Therefore, the disadvantages described above in relation to high temperature treatment also limit the use of this method.
There is also a method wherein the optical path of light beams in a solar cell is increase while the cell is kept as thin as possible. This method consists of providing a transparent layer on the upper cell surface. This layer is textured so that the light beams reaching it perpendicular to the cell plane are deviated by refraction and traverse the cell where its thickness is not minimal. Yet another method which achieves the same result consists of using a textured substrate and depositing layers of essentially uniform thickness on the substrate.